The present invention relates generally to systems for recording and/or reproducing audio signals which have been converted into digital signals. More particularly, it relates to systems which convert an analog audio signal into a digital signal and then record and/or reproduce this signal on/from a recording medium by inserting the digital signal between synchronizing signals, which correspond to the synchronizing signals of a composite video signal.
A known recording and reproducing apparatus for audio signals records and reproduces an analog audio signal, as it is originally produced, on and from a traveling magnetic tape. The recording and reproduction is accomplished by means of a fixed head, with unavoidably accompanying by problems such as wow and flutter of the tape travel, and noise, distortion, etc., due to the tape and head systems. As a consequence, these problems have constituted a barrier, limiting efforts to improve the quality of recording and reproducing of audio signals.
Accordingly, there is a method wherein an analog audio signal is converted into a digital signal by a method such as pulse code modulation (PCM). This method, greatly improves the S/N ratio, and reduces the effects such as distortion due to a nonlinearity of the recording medium.
This recording and reproducing of a digital signal requires a recording and reproducing system having a wider band or a greater number of channels than is required in a recording and reproducing system using an analog signal. Accordingly, there is a method using this recording and reproducing apparatus in a video tape recorder (hereinafter referred to as "VTR") which has heretofore been sold on the market and widely used as an apparatus for recording and/or reproducing a composite video signal.
As used herein the term "composite video signal" means a signal composed of an information video signal and synchronizing signals such as vertical synchronizing signals, equalizing pulses, and horizontal synchronizing signals. A VTR of this type is capable of recording/reproducing video signals in a wide band and on a track positioned obliquely on the traveling magnetic tape, by means of rotary heads which scan with a relatively high relative scanning speed.
To record an audio signal through the use of the VTR, an analog audio signal, to be recorded, is converted into a digital signal in an adapter device connected to the VTR. This digital signal is interposed between synchronizing signals which correspond to the synchronizing signals of an ordinary composite video signal. The signal thus obtained is fed to the VTR and recorded on the magnetic tape, by the rotary heads. At the time of reproducing, the signal reproduced from the magnetic tape by the rotary heads in the VTR is fed to the adapter device. There, the synchronizing signals are removed and the digital signal is reconverted into an analog signal and thus restored to the original audio signal.
In general, in order to obtain a large S/N ratio in a recording and reproducing operation in a VTR, a pre-emphasis circuit emphaized the high-frequency range of components, which is readily affected by noise. This pre-emphasis circuit is provided in the recording system. A de-emphasis circuit de-emphasizes the above mentioned high-frequency range of components, thus restoring it to its original state. This de-emphasis circuit is provided in the reproducing system. When a signal having relatively steep rising or falling portions is passed through this pre-emphasis circuit, sharp overshoots and undershoots occur at the instants of this rising and falling. Accordingly, in order to prevent over-modulation due to these overshoots and undershoots, a white clipping and dark clipping circuit is provided for clipping overshoots and undershoots exceeding respective specific values.
In the above mentioned system, according to the conventional practice, the analog audio signal to be recorded has been converted into a digital signal. A level equal to the white peak level of an ordinary video signal becomes "1", while a level equal to the pedestal level becomes "0". However, when a signal which has been converted into a digital signal in this manner passes through the pre-emphasis circuit of the VTR, very great overshoots and undershoots occur at the instants of rising and falling of the digital signal.
When the overshoots and undershoots are clipped by the above mentioned clipping circuits, the clipped signal differs from the original signal. As is apparent also from a Fourier expansion, the discrimination between "1" and "1" of the reproduced digital signal becomes impossible, whereby errors occur in some cases.